diff --git a/lib/SILGen/SILGen.cpp b/lib/SILGen/SILGen.cpp
index fe1cc4ea13e..678cf23997a 100644
--- a/lib/SILGen/SILGen.cpp
+++ b/lib/SILGen/SILGen.cpp
@@ -975,10 +975,12 @@ SILFunction *SILGenModule::emitLazyGlobalInitializer(StringRef funcName,
                                                  PatternBindingDecl *binding,
                                                      unsigned pbdEntry) {
   ASTContext &C = M.getASTContext();
-  Type initType = FunctionType::get(
-                    TupleType::getEmpty(C), TupleType::getEmpty(C),
-                    FunctionType::ExtInfo()
-                      .withRepresentation(FunctionType::Representation::Thin));
+  auto *onceBuiltin =
+      cast<FuncDecl>(getBuiltinValueDecl(C, C.getIdentifier("once")));
+  auto blockParam = onceBuiltin->getParameterLists()[0]->get(1);
+  auto *type = blockParam->getType()->castTo<FunctionType>();
+  Type initType = FunctionType::get(TupleType::getEmpty(C),
+                                    TupleType::getEmpty(C), type->getExtInfo());
   auto initSILType = getLoweredType(initType).castTo<SILFunctionType>();
 
   auto *f =
diff --git a/lib/SILOptimizer/IPO/GlobalOpt.cpp b/lib/SILOptimizer/IPO/GlobalOpt.cpp
index bccb8c05aaa..79124f2f56a 100644
--- a/lib/SILOptimizer/IPO/GlobalOpt.cpp
+++ b/lib/SILOptimizer/IPO/GlobalOpt.cpp
@@ -321,8 +321,8 @@ static SILFunction *getCalleeOfOnceCall(BuiltinInst *BI) {
 
   auto Callee = BI->getOperand(1);
   assert(Callee->getType().castTo<SILFunctionType>()->getRepresentation()
-           == SILFunctionTypeRepresentation::Thin &&
-         "Expected thin function representation!");
+           == SILFunctionTypeRepresentation::CFunctionPointer &&
+         "Expected C function representation!");
 
   if (auto *FR = dyn_cast<FunctionRefInst>(Callee))
     return FR->getReferencedFunction();
diff --git a/test/IRGen/globals.swift b/test/IRGen/globals.swift
index 9a5e68cedec..866961e58ba 100644
--- a/test/IRGen/globals.swift
+++ b/test/IRGen/globals.swift
@@ -54,5 +54,5 @@ extension A {
 // CHECK:      store  i64 {{.*}}, i64* getelementptr inbounds ([[INT]], [[INT]]* @_T07globals2g0Sivp, i32 0, i32 0), align 8
 
 // FIXME: give these initializers a real mangled name
-// CHECK: define internal swiftcc void @globalinit_{{.*}}func0() {{.*}} {
+// CHECK: define internal void @globalinit_{{.*}}func0() {{.*}} {
 // CHECK:      store i64 5, i64* getelementptr inbounds (%TSi, %TSi* @_T07globals1AV3fooSivpZ, i32 0, i32 0), align 8
diff --git a/test/IRGen/lazy_globals.swift b/test/IRGen/lazy_globals.swift
index 3354abc2437..28f8bef44ef 100644
--- a/test/IRGen/lazy_globals.swift
+++ b/test/IRGen/lazy_globals.swift
@@ -7,7 +7,7 @@
 // CHECK: @_T012lazy_globals1ySivp = hidden global %TSi zeroinitializer, align 8
 // CHECK: @_T012lazy_globals1zSivp = hidden global %TSi zeroinitializer, align 8
 
-// CHECK: define internal swiftcc void @globalinit_[[T]]_func0() {{.*}} {
+// CHECK: define internal void @globalinit_[[T]]_func0() {{.*}} {
 // CHECK: entry:
 // CHECK:   store i64 1, i64* getelementptr inbounds (%TSi, %TSi* @_T012lazy_globals1xSivp, i32 0, i32 0), align 8
 // CHECK:   store i64 2, i64* getelementptr inbounds (%TSi, %TSi* @_T012lazy_globals1ySivp, i32 0, i32 0), align 8
diff --git a/test/SILGen/lazy_globals.swift b/test/SILGen/lazy_globals.swift
index 4b184fbd848..8a7c0d47755 100644
--- a/test/SILGen/lazy_globals.swift
+++ b/test/SILGen/lazy_globals.swift
@@ -1,6 +1,6 @@
 // RUN: %target-swift-frontend -parse-as-library -emit-silgen -enable-sil-ownership %s | %FileCheck %s
 
-// CHECK: sil private @globalinit_[[T:.*]]_func0 : $@convention(thin) () -> () {
+// CHECK: sil private @globalinit_[[T:.*]]_func0 : $@convention(c) () -> () {
 // CHECK:   alloc_global @_T012lazy_globals1xSiv
 // CHECK:   [[XADDR:%.*]] = global_addr @_T012lazy_globals1xSivp : $*Int
 // CHECK:   store {{%.*}} to [trivial] [[XADDR]] : $*Int
@@ -8,15 +8,15 @@
 // CHECK: sil hidden [global_init] @_T012lazy_globals1xSivau : $@convention(thin) () -> Builtin.RawPointer {
 // CHECK:   [[TOKEN_ADDR:%.*]] = global_addr @globalinit_[[T]]_token0 : $*Builtin.Word
 // CHECK:   [[TOKEN_PTR:%.*]] = address_to_pointer [[TOKEN_ADDR]] : $*Builtin.Word to $Builtin.RawPointer
-// CHECK:   [[INIT_FUNC:%.*]] = function_ref @globalinit_[[T]]_func0 : $@convention(thin) () -> ()
-// CHECK:   builtin "once"([[TOKEN_PTR]] : $Builtin.RawPointer, [[INIT_FUNC]] : $@convention(thin) () -> ()) : $()
+// CHECK:   [[INIT_FUNC:%.*]] = function_ref @globalinit_[[T]]_func0 : $@convention(c) () -> ()
+// CHECK:   builtin "once"([[TOKEN_PTR]] : $Builtin.RawPointer, [[INIT_FUNC]] : $@convention(c) () -> ()) : $()
 // CHECK:   [[GLOBAL_ADDR:%.*]] = global_addr @_T012lazy_globals1xSivp : $*Int
 // CHECK:   [[GLOBAL_PTR:%.*]] = address_to_pointer [[GLOBAL_ADDR]] : $*Int to $Builtin.RawPointer
 // CHECK:   return [[GLOBAL_PTR]] : $Builtin.RawPointer
 // CHECK: }
 var x: Int = 0
 
-// CHECK: sil private @globalinit_[[T:.*]]_func1 : $@convention(thin) () -> () {
+// CHECK: sil private @globalinit_[[T:.*]]_func1 : $@convention(c) () -> () {
 // CHECK:   alloc_global @_T012lazy_globals3FooV3fooSivpZ
 // CHECK:   [[XADDR:%.*]] = global_addr @_T012lazy_globals3FooV3fooSivpZ : $*Int
 // CHECK:   store {{.*}} to [trivial] [[XADDR]] : $*Int
@@ -26,8 +26,8 @@ struct Foo {
 // CHECK: sil hidden [global_init] @_T012lazy_globals3FooV3fooSivau : $@convention(thin) () -> Builtin.RawPointer {
 // CHECK:   [[TOKEN_ADDR:%.*]] = global_addr @globalinit_[[T]]_token1 : $*Builtin.Word
 // CHECK:   [[TOKEN_PTR:%.*]] = address_to_pointer [[TOKEN_ADDR]] : $*Builtin.Word to $Builtin.RawPointer
-// CHECK:   [[INIT_FUNC:%.*]] = function_ref @globalinit_[[T]]_func1 : $@convention(thin) () -> ()
-// CHECK:   builtin "once"([[TOKEN_PTR]] : $Builtin.RawPointer, [[INIT_FUNC]] : $@convention(thin) () -> ()) : $()
+// CHECK:   [[INIT_FUNC:%.*]] = function_ref @globalinit_[[T]]_func1 : $@convention(c) () -> ()
+// CHECK:   builtin "once"([[TOKEN_PTR]] : $Builtin.RawPointer, [[INIT_FUNC]] : $@convention(c) () -> ()) : $()
 // CHECK:   [[GLOBAL_ADDR:%.*]] = global_addr @_T012lazy_globals3FooV3fooSivpZ : $*Int
 // CHECK:   [[GLOBAL_PTR:%.*]] = address_to_pointer [[GLOBAL_ADDR]] : $*Int to $Builtin.RawPointer
 // CHECK:   return [[GLOBAL_PTR]] : $Builtin.RawPointer
@@ -40,7 +40,7 @@ struct Foo {
   static var initialized: Int = 57
 }
 
-// CHECK: sil private @globalinit_[[T:.*]]_func3 : $@convention(thin) () -> () {
+// CHECK: sil private @globalinit_[[T:.*]]_func3 : $@convention(c) () -> () {
 // CHECK:   alloc_global @_T012lazy_globals3BarO3barSivpZ
 // CHECK:   [[XADDR:%.*]] = global_addr @_T012lazy_globals3BarO3barSivpZ : $*Int
 // CHECK:   store {{.*}} to [trivial] [[XADDR]] : $*Int
@@ -50,8 +50,8 @@ enum Bar {
 // CHECK: sil hidden [global_init] @_T012lazy_globals3BarO3barSivau : $@convention(thin) () -> Builtin.RawPointer {
 // CHECK:   [[TOKEN_ADDR:%.*]] = global_addr @globalinit_[[T]]_token3 : $*Builtin.Word
 // CHECK:   [[TOKEN_PTR:%.*]] = address_to_pointer [[TOKEN_ADDR]] : $*Builtin.Word to $Builtin.RawPointer
-// CHECK:   [[INIT_FUNC:%.*]] = function_ref @globalinit_[[T]]_func3 : $@convention(thin) () -> ()
-// CHECK:   builtin "once"([[TOKEN_PTR]] : $Builtin.RawPointer, [[INIT_FUNC]] : $@convention(thin) () -> ()) : $()
+// CHECK:   [[INIT_FUNC:%.*]] = function_ref @globalinit_[[T]]_func3 : $@convention(c) () -> ()
+// CHECK:   builtin "once"([[TOKEN_PTR]] : $Builtin.RawPointer, [[INIT_FUNC]] : $@convention(c) () -> ()) : $()
 // CHECK:   [[GLOBAL_ADDR:%.*]] = global_addr @_T012lazy_globals3BarO3barSivpZ : $*Int
 // CHECK:   [[GLOBAL_PTR:%.*]] = address_to_pointer [[GLOBAL_ADDR]] : $*Int to $Builtin.RawPointer
 // CHECK:   return [[GLOBAL_PTR]] : $Builtin.RawPointer
@@ -63,13 +63,13 @@ enum Bar {
 
 func f() -> (Int, Int) { return (1, 2) }
 
-// CHECK: sil private @globalinit_[[T]]_func4 : $@convention(thin) () -> () {
+// CHECK: sil private @globalinit_[[T]]_func4 : $@convention(c) () -> () {
 // CHECK:   function_ref @_T012lazy_globals1fSi_SityF : $@convention(thin) () -> (Int, Int)
 // CHECK: sil hidden [global_init] @_T012lazy_globals2a1Sivau : $@convention(thin) () -> Builtin.RawPointer
-// CHECK:   function_ref @globalinit_[[T]]_func4 : $@convention(thin) () -> ()
+// CHECK:   function_ref @globalinit_[[T]]_func4 : $@convention(c) () -> ()
 // CHECK:   global_addr @_T012lazy_globals2a1Sivp : $*Int
 // CHECK: sil hidden [global_init] @_T012lazy_globals2b1Sivau : $@convention(thin) () -> Builtin.RawPointer {
-// CHECK:   function_ref @globalinit_[[T]]_func4 : $@convention(thin) () -> ()
+// CHECK:   function_ref @globalinit_[[T]]_func4 : $@convention(c) () -> ()
 // CHECK:   global_addr @_T012lazy_globals2b1Sivp : $*Int
 var (a1, b1) = f()
 
diff --git a/test/SILGen/properties.swift b/test/SILGen/properties.swift
index a6919a7b698..c9722dee915 100644
--- a/test/SILGen/properties.swift
+++ b/test/SILGen/properties.swift
@@ -627,7 +627,7 @@ func force_global_observing_property_setter() {
 }
 
 // The property is initialized with "zero".
-// CHECK-LABEL: sil private @globalinit_{{.*}}_func1 : $@convention(thin) () -> () {
+// CHECK-LABEL: sil private @globalinit_{{.*}}_func1 : $@convention(c) () -> () {
 // CHECK: bb0:
 // CHECK-NEXT: alloc_global @_T010properties25global_observing_propertySiv
 // CHECK-NEXT: %1 = global_addr @_T010properties25global_observing_propertySivp : $*Int
diff --git a/test/SILOptimizer/globalopt.sil b/test/SILOptimizer/globalopt.sil
index 3a1a6437440..a6c22a386f9 100644
--- a/test/SILOptimizer/globalopt.sil
+++ b/test/SILOptimizer/globalopt.sil
@@ -213,7 +213,7 @@ sil_global private @globalinit_token0 : $Builtin.Word
 sil_global @MyConst : $Int32
 
 // globalinit_func0
-sil private @globalinit_func0 : $@convention(thin) () -> () {
+sil private @globalinit_func0 : $@convention(c) () -> () {
 bb0:
   %0 = global_addr @MyConst : $*Int32               // user: %3
   %1 = integer_literal $Builtin.Int32, 0           // user: %2
@@ -229,8 +229,8 @@ bb0:
   %1 = global_addr @globalinit_token0 : $*Builtin.Word // user: %2
   %2 = address_to_pointer %1 : $*Builtin.Word to $Builtin.RawPointer // user: %5
   // function_ref globalinit_func0
-  %3 = function_ref @globalinit_func0 : $@convention(thin) () -> () // user: %4
-  %5 = builtin "once"(%2 : $Builtin.RawPointer, %3 : $@convention(thin) () -> ()) : $()
+  %3 = function_ref @globalinit_func0 : $@convention(c) () -> () // user: %4
+  %5 = builtin "once"(%2 : $Builtin.RawPointer, %3 : $@convention(c) () -> ()) : $()
   %6 = global_addr @MyConst : $*Int32               // user: %7
   %7 = address_to_pointer %6 : $*Int32 to $Builtin.RawPointer // user: %8
   return %7 : $Builtin.RawPointer                 // id: %8
diff --git a/test/SILOptimizer/static_initializer.sil b/test/SILOptimizer/static_initializer.sil
index d808471d54e..40d26ac1dd7 100644
--- a/test/SILOptimizer/static_initializer.sil
+++ b/test/SILOptimizer/static_initializer.sil
@@ -25,8 +25,8 @@ sil_global private @globalinit_token0 : $Builtin.Word
 // CHECK-NEXT: }
 sil_global @_Tv2ch1xSi : $Outer
 
-// CHECK-LABEL: sil private @globalinit_func0 : $@convention(thin) () -> () {
-sil private @globalinit_func0 : $@convention(thin) () -> () {
+// CHECK-LABEL: sil private @globalinit_func0 : $@convention(c) () -> () {
+sil private @globalinit_func0 : $@convention(c) () -> () {
 bb0:
   %0 = global_addr @_Tv2ch1xSi : $*Outer
   %1 = integer_literal $Builtin.Int32, 2
@@ -42,7 +42,7 @@ bb0:
 // CHECK-NEXT: bb0:
 // CHECK-NOT: global_addr @globalinit_token0
 // CHECK-NEXT: address_to_pointer
-// CHECK: function_ref @globalinit_func0 : $@convention(thin) () -> ()
+// CHECK: function_ref @globalinit_func0 : $@convention(c) () -> ()
 // CHECK-NEXT: global_addr @_Tv2ch1xSi : $*Outer
 // CHECK-NEXT: address_to_pointer
 // CHECK-NEXT: return
@@ -50,8 +50,8 @@ sil [global_init] @_TF2cha1xSi : $@convention(thin) () -> Builtin.RawPointer {
 bb0:
   %1 = global_addr @globalinit_token0 : $*Builtin.Word
   %2 = address_to_pointer %1 : $*Builtin.Word to $Builtin.RawPointer
-  %3 = function_ref @globalinit_func0 : $@convention(thin) () -> ()
-  %5 = builtin "once"(%2 : $Builtin.RawPointer, %3 : $@convention(thin) () -> ()) : $()
+  %3 = function_ref @globalinit_func0 : $@convention(c) () -> ()
+  %5 = builtin "once"(%2 : $Builtin.RawPointer, %3 : $@convention(c) () -> ()) : $()
   %6 = global_addr @_Tv2ch1xSi : $*Outer
   %7 = address_to_pointer %6 : $*Outer to $Builtin.RawPointer
   return %7 : $Builtin.RawPointer